The present invention relates to a method of forming a tunnel oxide layer, and more particularly, to a method of forming a tunnel oxide layer by utilizing the rapid thermal oxidation (RTO) method and annealing the tunnel oxide layer in-situ by utilizing the rapid thermal annealing (RTA) method.
Generally, according to the difference in accessing functions, the memory devices can be divided into random access memory (RAM) and read only memory (ROM), wherein RAM must keep the supplying power uninterruptedly for reserving the stored data and is thus called as a volatile memory, and ROM does not lose the stored data even with the break of supplying power and is thus called as a nonvolatile memory. In addition, according to the varieties of the ways for storing data, ROM can be further divided into mask read only memory (MROM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), and flash memory, etc. Because the data stored in a non-volatile memory can be kept after the power is shut off, non-volatile memory device is applied widely in the industries of computer and electronics. Particularly, as the increasing popularization of portable electric devices, such as notebook computer and telecommunication equipment, etc., and the increasing device integration, the technical demands for electrically erasable programmable ROM and flash memory, which can be accessed like disk drives, is increasing day by day.
Referring to FIG. 1, it shows a cross-sectional view of a flash memory cell. In the flash memory cell, a tunnel oxide layer 16, floating gate 18, dielectric layer 20, and control gate 22 are stacked in order on a P-type substrate 10, and the N-type drain 12 and source 14 are formed under the upper surface of substrate 10, wherein the composition of tunnel oxide layer 16 can be, for example, silicon dioxide (SiO2), and the composition of the floating gate 18 and control gate 22 can be, for example, poly-silicon, and the composition of dielectric layer 20 can be, for example, silicon dioxide or silicon nitride (Si3N4).
If the source 14 and the substrate 10 are connected to ground, and the control gate 22 and the drain 12 are applied with high-voltage electricity, the carrier multiplication phenomenon will occur between the N-type drain 12 and the P-type substrate 10. A portion of the hot electrons resulted from the carrier multiplication phenomenon are absorbed by the drain 12, and the other potion of them pass through the tunnel oxide layer 16 and into the floating gate 18, so as to make the floating gate 18 charged. Because there is a potential barrier in the composition of tunnel oxide layer 16 and dielectric layer 20 that connect to the floating gate 18, the electrons within the floating gate 18 cannot escape but stay in the floating gate 18. While a voltage is applied to the control gate 22 for accessing the data in the aforementioned flash memory cell, with the charged floating gate 18, data xe2x80x9c1xe2x80x9d is stored. On the other hand, with no electron staying within the floating gate 18, data xe2x80x9c0xe2x80x9d stored in the flash memory cell. As to deleting the data stored in memory cell, by applying an appropriate negative voltage to the control gate 22, the electrons within floating gate 18 are induced to leave floating gate 18 through the tunnel oxide layer 16, and thereby the data stored in the flash memory cell is deleted.
In the memory cells of EEPROM and flash memory, the written/erased action of data are performed by prompting hot electrons passing through the tunnel oxide layer to get enter/exit from the floating gate. Thus, the quality of electricity of the tunnel oxide layer has great influence on the stability of memory device, and the reliability of device is lowered by many impurities or charges, and the unnecessary variation of electricity is induced.
The stability of electricity of thermal oxide layer, such as tunnel oxide layer, etc., is influenced principally by the increase of charge concentration of oxide layer resulted from the trapped charges induced during the operation of device by impurity defects or unsaturated bonding of the interface between silicon substrate and oxide layer, so that the charges are kept within the oxide layer. In order to lower the charge concentration in the oxide layer, after the oxide layer is formed, an annealing process is used to lower the concentration of impurity defects or unsaturated bonding, so that the amount of trapped charge is decreased, and the charge concentration of oxide layer is lowered.
Generally, most of the silicon dioxide for the tunnel oxide layer is formed and annealed with the use of a thermal oxidation furnace by the furnace process. An annealing process is a kind of metal smelting technique in wide application, and the annealing principle is to use thermal energy to increase the energy of lattice atoms and material defects, so that vibration and diffusion of lattice atoms and defects are increased, and the arrangement of atoms of material is rearranged, and the material defect of are lowered. In virtue of the disappearance of defects, recrystallization is performed, and further grain growth is performed. The purpose of the annealing process is to eliminate the material defects and rearrange the structure of material, so after the oxide layer is formed, the annealing process can be applied to eliminate the defects of oxide layer and enhance the quality of electricity of oxide layer.
Referring to FIG. 2, FIG. 2 shows a cross-sectional view of a conventional thermal oxidation furnace for forming and annealing the tunnel oxide layer. Thermal oxidation furnace 100 is mainly composed of a quartz tube 102 that has been annealed by high-temperature heating, a heater 104, and thermocouples (not shown) used for measuring the furnace tube temperatures.
For forming a silicon dioxide film served as the tunnel oxide layer with the use of a thermal oxidation furnace 100, about from 100 to 150 pieces of substrate 106 are first put on boats 112 made of, for example, quartz, and then into a thermal oxidation furnace 100, wherein an appropriate amount of nitrogen (N2) is injected into the thermal oxidation furnace 100 through a gas inlet 108. Then, after the temperature of thermal oxidation furnace 100 is raised, oxygen (O2) and hydrogen (H2) followed are injected into the thermal oxidation furnace 100 to form a silicon dioxide film by thermal oxidation, wherein for the prevention of hydrogen explosion caused by the hydrogen accumulated in the thermal oxidation furnace 100, the amount of oxygen must be at least greater than a half of the amount of hydrogen, and the gas after reaction has to be exhausted through the gas outlet 110. Then, hydrogen and oxygen are stopped being injected into the thermal oxidation furnace 100, and the system temperature is lowered with the use of nitrogen.
Subsequently, the thermal oxidation furnace 100 is used to anneal the tunnel oxide layer, and a reactive gas, such as nitrogen, is injected into the thermal oxidation furnace 100 through the gas inlet 108. The temperature of the thermal oxidation furnace 100 is raised to an appropriate high temperature by the use of heater 104, and substrate 106 is placed at the high-temperature environment for a period of time. At this time, the lattices of the atoms of the tunnel oxide layer on substrate 106 are rearranged with the thermal energy resulting from the high temperature. Subsequently, after the temperature of the thermal oxidation furnace 100 is lowered, substrate 106 is taken out of the thermal oxidation furnace 100, and the process of forming the tunnel oxide layer is completed.
Although more than one hundred of substrates can be treated simultaneously by forming and annealing silicon dioxide film with the use of furnace process for performing thermal process, yet it takes quite a long time for the process to be completed, which is around several hours, and thus not only the process efficiency is lowered but also the thermal budget is enhanced. Consequently, the process cost is increased.
According to the aforementioned conventional method for forming and annealing a tunnel oxide layer, while a silicon dioxide film is formed to be the tunnel oxide layer by utilizing the furnace process, for it takes a lot of time for completing the process, which results in the lower process efficiency, and the overwhelming process thermal budget, the process cost is increased.
Accordingly, one of major objects of the present invention is to provide a method of forming a tunnel oxide layer, and the method of the present invention is to utilize the rapid thermal process to form a silicon dioxide film served as the tunnel oxide layer on single wafer, so as to reduce the reactive time and thus enhance the throughput.
Another object of the present invention is to provide a method of forming a tunnel oxide layer, and the method of the present invention is to use the rapid thermal process to form the silicon dioxide film served as the tunnel oxide layer on single wafer by rapid thermal oxidation. Hence, not only the growth rate of silicon dioxide film is increased, but also the process efficiency is enhanced, and further the quality of the film is more uniformed.
A further object of the present invention is to provide a method of annealing a tunnel oxide layer, and the method of the present invention is to use the rapid thermal oxidation process to form a tunnel oxide on a single wafer, and in the same chamber use the rapid thermal annealing process to anneal the tunnel oxide in-situ. Thus, the reactive time can be reduced, and the process thermal budget can be lowered significantly, so that the production cost can be reduced. Further, the quality of the tunnel oxide layer can be enhanced, and not only the contamination but also the consumed manpower and time resulted from changing chamber can be avoided.
Based on the objects decreased above, the present invention is to provide a method of forming a tunnel oxide layer, and the method of the present invention is to utilize the rapid thermal oxidation of the rapid thermal process. Firstly, substrate are taken into a rapid heater, after the temperature is adjusted to the preset value, nitrogen is injected into the rapid heater. Then, a first reactive temperature of rapid heater is raised to between about 850xc2x0 C. and about 1100xc2x0 C., and a first reactive gas comprising oxygen and hydrogen is injected into the rapid heater to perform the thermal oxidation reaction, wherein the flow ratio of hydrogen to the combination of hydrogen and oxygen is adjusted to between about 1% and about 33%, and the process pressure is maintained between about 5 torr and about 15 torr. After the thermal oxidation reaction is completed, the supply of hydrogen and oxygen are stopped, and the temperature of the rapid heater is decreased rapidly to the preset value with the use of nitrogen.
Subsequently, a second reactive gas comprising nitrogen and oxygen, is injected into the rapid heater, wherein the flow rate of nitrogen is between about 5 sccm and about 10 sccm, and the flow rate of oxygen is between about 0.2 sccm and about 0.5 sccm. Then, a second reactive temperature of the rapid heater is raised to between about 850. and about 1100., and the process pressure is maintained between about 700 torr and about 760 torr, and the substrate is maintained under the second reactive temperature for approximately dozens of seconds. During this period, the lattice positions of the atoms of the tunnel oxide layer on the substrate are rearranged by utilizing thermal energy resulting from high temperature. Subsequently, the temperature of the rapid heater is decreased rapidly to the preset value, the substrate is taken out, and the process for forming the tunnel oxide layer is completed. With the rapid heater of the rapid thermal process, the chamber of the rapid thermal process can be heated to the temperature required for the thermal process in seconds, and after the reaction is completed, the temperature can then be lowered to the preset value at an amazing speed. With the application of the present invention, the processing time is quite short, thus it not only has advantages of lowering thermal budget, lowering cost and increasing process throughput, but also a tunnel oxide layer with better uniformity is further obtained.